The processing of silicon wafers to produce integrated circuits generally includes one or more stages of wafer heating. One approach for wafer heating is Rapid Thermal Processing (RTP), which is presently used for a variety of applications including silicidation, oxide formation, and anneal processes. A typical RTP system includes a heating chamber in which a wafer is positioned for thermal processing. High-intensity heating lamps are controlled for generating radiant heat for heating the wafer, and temperature sensing equipment is provided for sensing the temperature in the chamber and/or for feedback-control of the heating lamps. The wafer is rapidly heated and cooled (e.g., 50° to 250° C./sec), usually maintaining a peak temperature (e.g., 150° to 1200° C.) for a short period of time (e.g., 10 to 120 sec) according to a prescribed RTP recipe.
An important factor in the thermal processing of wafers is maintaining temperature uniformity within the wafer. A temperature difference across a wafer can cause Vt variations within the wafer and/or stress that leads to device damage, resulting in decreased product yield. For this reason, temperature uniformity during RTP is a subject of considerable interest, and Temperature uniformity during RTP is often inferred from the sheet-resistance uniformity of the wafer post-processing. Sheet resistance varies according to the temperature to which the wafer is heated, so variations in sheet resistance can serve as an accurate indicator of variations in temperature that occurred during RTP.
One of the ways of monitoring the annealing temperature within the heating chamber is to calibrate the heating chamber with a thermal coupled standard consisting of temperature sensors that display the exact temperature depending on the location of these sensors while the thermal coupled standard is seated inside the heating chamber. However, thermal coupled standard monitor and calibration cannot be performed while wafers are being processed inline, and often requires the RTP system be taken offline in order to perform the calibrations which can be costly as well as time consuming.
A more time and cost efficient method of monitoring the annealing temperature uniformity within the heating chamber is to use the Rs-49 point contour map. The Rs-49 point contour map method assesses wafer heating uniformity by measuring the sheet resistance (Rs) of 49 uniformly distributed points across the semiconductor substrate after the semiconductor substrate has been annealed inside the heating chamber. The measurements can be done on batches of processed wafers and provide a quick and accurate assessment of the annealing temperature uniformity by plotting a contour map of the Rs values based on their position on the semiconductor substrate. The measurements can then be used to calibrate or fine-tune the RTP system for improving heating uniformity. However, while the Rs-49 point contour map method is an efficient method to monitor and fine tune RTP systems, there remains considerable room for improvement. Thus, there exists a need for improved techniques for calibrating or fine-tuning RTP systems in order to provide for more ideal heating uniformity during RTP processes.